One of the important applications for compact optical devices is in the field of HMDs, wherein an optical module serves both as a reflecting optical element and a combiner, in which a two-dimensional display is imaged to infinity and reflected into the eye of an observer. The display can be obtained directly from either a spatial light modulator (SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode array (OLED), a scanning source or similar devices, or indirectly, by means of a relay lens or an optical fiber bundle. The display comprises an array of elements (pixels) imaged to infinity by a collimating lens and transmitted into the eye of the viewer by means of a reflecting or partially reflecting surface acting as a combiner for non-see-through and see-through applications, respectively. Typically, a conventional, free-space optical module is used for these purposes. As the desired field-of-view (FOV) of the system increases, such a conventional optical module becomes larger, heavier, bulkier, and therefore, even for a moderate performance device, is impractical. This is a major drawback for all kinds of displays, but especially in head-mounted applications, where the system must necessarily be as light and as compact as possible.
The strive for compactness has led to several different complex optical solutions, all of which, on the one hand, are still not sufficiently compact for most practical applications, and, on the other hand, suffer major drawbacks in terms of manufacturability. Furthermore, the eye-motion-box (EMB) of the optical viewing angles resulting from these designs is usually very small—typically, less than 6 mm. Hence, the performance of the optical system is sensitive, even for small movements of the optical system relative to the eye of the viewer, and does not allow sufficient pupil motion for comfortable reading of text from such displays.
The teachings included in the Publications WO 01/95027, WO 2006/013565, WO 2006/085309, WO 2006/085310, WO 2007/054928 and WO 2008/023367 in the name of Applicant, are herein incorporated by references.